The present invention relates generally to communication systems, and more particularly to method and apparatus for processing DS3 (Digital Signal Level 3) signals.
DS3 is a popular bit serial protocol for carrying voice and data signals over a single transmission facility. A DS3 frame contains 7 rows and 8 columns for each row where each column or block stores 85 bits of data. A shortcoming of DS3 protocol is that the bit alignment within a DS3 frame will rotate around the byte boundaries, which presents challenges to designers in the implementation of the DS3 bit serial protocol. The logic circuits in a DS3 processing can be simplified significantly if every DS3 frame can be arranged so that the overhead bit is placed at the most significant bit position in a shorter and efficient data format.
For additional background information on DS3 framing, the reader is referred to the DS3 standard as adopted in ANSI T1.107 (1995). The content of this document is incorporated herein by reference.
Accordingly, it is desirable to have a method and system for processing DS3 frames with a more efficient approach in simplifying the complexity of the implementation.
The present invention overcomes the foregoing limitations by disclosing a pentad-base system, comprising (1) a first converter having an input for receiving an input data having a specified format, the first converter converting the input data to a pentad, the first converter having an output for generating the pentad; (2) a framer having an input and an output, the framer determining the location of an overhead bit in the pentad and generating a control signal; (3) a barrel shifter having a first input coupled to the output of said first converter, a second input coupled to the output of said framer, and an output coupled to the input of said framer, said barrel shifter receiving a control signal from the framer indicating the number of shift to place the overhead bit at the most significant bit position, said barrel shifter shifting an overhead bit to the most significant bit position.
The pentad-based system in the present invention advantageously affixes the location of the overhead bit so that the column and row boundary alignments within a DS3 frame is clearly known. After a framer has found F-bits, the barrel shifter shifts the overhead bit to the most significant bit location of a pentad. Then the framer searches for the M-bits at the most significant location of the pentads, and once the framer has found the M-bit alignment, the framer declares in-Frame condition. The placement of the overhead bit in a DS3 frame at the most significant bit position simplifies the designs and verifications of DS3 processing.